Tests, written before the change, for the PIXA_ variables and PORT:
every key set from the environment with no config file, PORT over the
file's port, a list variable replacing the file's list, an empty
variable as a set value, invalid values aborting startup naming the
variable, and the signing key and metrics password never printed. All
fail until the change lands, except the check that a config file alone
behaves as before. TestMain unsets PORT and every PIXA_ variable so the
shell running the tests cannot change their result.
Model: opus-5-5
